Sandwich type immunoassays in which there is immobilized to a solid substrate a first antibody which is specific for a first epitope of an analyte whose presence or concentration is being sought and a second labeled antibody specific for a second epitope of the analyte are well known. This type of assay can be run in microtitre plates or in a strip format where the fluid being tested for the analyte is applied to one end of a porous strip and allowed to flow through the various regions of the strip by capillarity. The analyte is bound to the immobilized antibody and the labeled antibody to complete the "sandwich". The label, referred to herein as the "signal generator" can be an enzyme which is reactive with a substrate to provide a colored response. Chromophores and fluorofores are also known signal generators. In U.S. Pat. No. 4,703,017 there is disclosed an immunoassay technique in which the signal generator is a particulate label which can be a liposome or microcapsule containing a visible dye which is visually detectable upon being bound to the first immobilized antibody via the analyte. Other signal generators include colloidal sized metal particles, such as gold sol, to which antibodies are bound.
The signal generated by the label on the analyte bound antibody is not always as strong as might be desired, and methods to amplify the signal have been proposed One such method, disclosed in U.S. Pat. No. 4,657,853 involves a method for producing a polymericenzyme/antibody conjugate by covalently coupling at least two enzyme molecules to produce a prepolymerized enzyme and then covalently coupling the prepolymerized enzyme to an antibody or antibody fragment.
In EP 0 516 529 A.sub.2, there is disclosed an agglutination type immunoassay in which an antibody having three or more binding sites to a specific antibody being assayed to thereby amplify the agglutination so that the detection sensitivity for the antibody is improved.
In published Japanese application 5-322893 there is disclosed an amplification system in which antibodies specific for the analyte and antibodies specific for a different epitope of the analyte bearing enzymes as the signal generator are combined in solution. In addition there are included antibodies whose Fab portions are each specific for the enzyme signal generator which facilitate the formation of chains of the enzyme bearing antibodies which result in signal amplification when the chain of enzyme labeled antibodies is contacted with a suitable substrate.